Energy efficient coatings are becoming increasingly used on architectural and automotive glass and in other applications. Their use is progressively mandated by government standards and the coatings are becoming quite sophisticated as the specifications they must meet for control of solar transmission, infrared transmission and heat retention become ever more demanding.
Indeed, in order to meet the demands for improved energy efficiency, it has been found necessary to deposit at least some of the coatings as multilayer interference stacks. (ref. Coated Glass Applications and Markets, R. Hill and S. Nadel, published by BOC Coating Technology, Fairfield Calif., 1999, the entire contents of which are incorporated herein by reference). These stacks have a specific reflectance color when viewed at near-normal incidence. Such coated glass is invariably used as part of a double-paned window unit consisting of two lights. The lights are sealed into the window frame with a dry gas occupying the space between the lights. The coated surface of architectural glass is usually on the second surface of such a double-paned unit if the surfaces are counted from the solar side inward. The glass of the exterior light may be tinted and, as already noted, the coatings themselves usually have some color.
When such double-paned windows (known to the trade as insulated glass units or IGUs) are used as the external cladding of a large multi-story building, the architect and others wish to see a uniform reflected color from all angles. Unfortunately, for reasons well known to the designers of multilayer coating stacks, the reflectance color of such stacks can change quite perceptibly with viewing angle. If due attention is not paid to this aspect the color change can be quite dramatic and unacceptable. This change in color, as a function of viewing angle, is referred to herein as the angular color variation.
When windows are viewed from the exterior of a building, the angular color variation is more noticeable under light from cloudy sky conditions. The illumination from a cloudy sky is randomly polarized light and its energy is fairly evenly distributed through the visible spectrum (white light). These two light characteristics enhance the perception of reflected color and reflected color change with angle in fenestration products.
The color of light reflected from windows perceived by a viewer is the sum of reflections from all the reflecting surfaces contained within the window unit.
Specular reflections off any surface such as those within a window unit are partially polarized if viewed from any angle other than normal incidence. It is well known in the field of optics that polarization effects increase with reflected angle until the Brewster angle is reached. Polarization of reflected light then tends to decrease beyond the Brewster angle until at grazing incidence, polarization effects approach zero. The human eye is typically insensitive to polarization and is able to discern color free of polarization error.
In many instances, the control of angular color variation is managed by visual inspection against a limited number of samples. The chief disadvantage of this method is that it relies on subjective judgment of a color match which is often perceived differently by different inspectors in part because about 5% of the male population has some red/green color vision deficiency.